This invention relates generally to a method for spatially and temporally modulating a light beam and more specifically to imaging a modulated light onto a light sensitive media.
Photographic images are traditionally printed on photographic paper using conventional, film based optical printers. The photographic industry, however, is converting to digital imaging. One step in the digital printing process is to use images obtained from digital cameras, or scanned film exposed in traditional photographic cameras, to create digital image files that are then printed onto photographic paper.
The growth of the digital printing industry has led to multiple approaches to digital printing. One of the early methods used for digital printing was cathode ray tube (CRT) based printers such as the Centronics CRT recorder. This technology has several limitations related to the phosphor and the electron beam. The resolution of this technology is inadequate when printing a large format image, such as 8 inch by 10 inch photographic print. CRT printers also tend to be expensive, which is a severe shortcoming in a cost sensitive market. An additional limitation is that CRT printers do not provide sufficient red exposure to the media when operating at frame rates above 10,000 prints per hour.
Another commonly used approach to digital printing is the laser-based engine shown in U.S. Pat. No. 4,728,965. Such systems are generally polygon flying spot systems, which use red, green, and blue lasers. Unfortunately, as with CRT printers, the laser based systems tend to be expensive, since the cost of blue and green lasers remains quite high. Additionally, the currently available lasers are not compact. Another problem with laser based printing systems is that the photographic paper used for traditional photography is not suitable for a color laser printer due to reciprocity failure. High intensity reciprocity failure is a phenomenon by which photographic paper is less sensitive when exposed to high light intensity for a short period. For example, flying spot laser printers expose each of the pixels for a fraction of a microsecond, whereas optical printing systems expose the paper for the duration of the whole frame time, which can be on the order of seconds. Thus, a special paper is required for laser printers.
A more contemporary approach uses a single spatial light modulator such as a Texas Instruments digital micromirror device (DMD) as shown in U.S. Pat. No. 5,061,049. Spatial light modulators provide significant advantages in cost as well as allowing longer exposure times, and have been proposed for a variety of different printing systems from line printing systems such as the printer depicted in U.S. Pat. No. 5,521,748, to area printing systems such as the system described in U.S. Pat. No. 5,652,661.
One approach to printing using the Texas Instruments DMD, shown in U.S. Pat. No. 5,461,411, offers advantages such as longer exposure times using light emitting diodes (LED) as a source. See U.S. Pat. No. 5,504,514. However, this technology is not widely available. As a result, DMDs are expensive and not easily scaleable to higher resolution. Also, the currently available resolution is not sufficient for all printing needs.
Another low cost solution uses Liquid Crystal Display (LCD) modulators. Several photographic printers using this commonly available technology are described in U.S. Pat. Nos. 5,652,661, 5,701,185, and 5,745,156. Most of these designs involve the use of a transmissive LCD modulator such as is depicted in U.S. Pat. Nos. 5,652,661 and 5,701,185. While such methods offer several advantages in ease of optical design for printing, there are several drawbacks to the use of conventional transmissive LCD technology. Transmissive LCD modulators generally have reduced aperture ratios and the use of Transmissive Field-Effect-Transistors (TFT) on glass technology does not promote the pixel to pixel uniformity desired in many printing applications. Furthermore, in order to provide large numbers of pixels, many high resolution transmissive LCDs possess footprints of several inches. Such a large footprint can be unwieldy when combined with a print lens. As a result, most LCD printers using transmissive technology are constrained to either low resolution or small print sizes.
To print high resolution 8 inch by 10-inch images with at least 300 pixels per inch requires 2400 by 3000 pixels. Transmissive LCD modulators with such resolutions are not readily available. Furthermore, each pixel must have a gray scale depth to render a continuous tone print uniformly over the frame size, which is not available with this technology.
An alternate approach is to utilize reflective LCD modulators, which are widely accepted in the display market. Most of the activity in reflective LCD modulators has been related to projection display. The projectors are optimized to provide maximum luminous flux to the screen with secondary emphasis placed on contrast and resolution. To achieve the goals of projection display, most optical designs use high intensity lamp light sources. Additionally, many projector designs use three reflective LCD modulators, one for each of the primary colors, such as the design shown in U.S. Pat. No. 5,743,610. Using three reflective LCD modulators is both expensive and cumbersome.
For projectors using a single reflective LCD modulator, color sequential operation is required. To maintain the high luminosity in combination with the color sequential requirements, a rotating color filter wheel is sometimes employed. This is yet another large, moving part, which further complicates the system.
The recent advent of high resolution reflective LCDs with high contrast, greater than 100:1, presents possibilities for printing that were previously unavailable. See U.S. Pat. Nos. 5,325,137 and 5,805,274. Specifically, a printer may be based on a reflective LCD modulator illuminated sequentially by red, green, and blue light emitting diodes. The reflective LCD modulator may be sub-apertured and dithered in two or three directions to increase the resolution.
Dithering has been applied to transmissive LCD systems due to the less than perfect fill factor. Incorporating dithering into a reflective LCD printing system would allow high resolution printing while maintaining a small footprint. Also, because the naturally high fill factor present in many reflective LCD technologies, the dithering can be omitted with no detriment to the continuity of the printed image. While devices such as the Texas Instruments DMD can incorporate a secondary mask as shown in U.S. Pat. No. 5,754,217, the mask is displaced in some embodiments of the device, which makes manufacturing more difficult and adds to the processing complexity of the device.
Alternative forms of optical dithering are used to improve resolution in display systems incorporating LCD modulators. A calcite crystal or other electro-optic birefringent material can be used to optically shift the path of the image beam, where the amount of shift is dependent on the polarization characteristics of the image beam. This allows the shifting of one component of the image with respect to a second component of the image that has a different polarization. See U.S. Pat. No. 5,715,029 and 5,727,860. In addition to the use of birefringent material, U.S. Pat. No. 5,626,411 uses the law of refraction with isotropic optical media of different indices of refraction to displace one image component from a second image component. These methods of beam displacement are used in a dynamic imaging system and serve to increase resolution by interlacing raster lines to form two lines of sub-images. The two sub-images are imaged faster than in perceivable by the human visual system, so that the individual images are integrated into a composite image as seen by the observer.
The use of a reflective LCD serves to significantly reduce the cost of the printing system. Furthermore, the use of an area reflective LCD modulator sets the exposure times at sufficient length to avoid or significantly reduce reciprocity failure.
The progress in the reflective LCD device field made in response to needs of the projection display industry have provided opportunities in printing applications. Thus, a reflective LCD modulator designed for projection display can be incorporated into the printing design with little modification to the LCD itself. Also, by designing an exposure system and data path with an existing projection display device allows incorporation of an inexpensive commodity item into the print engine.
Of the reflective LCD technologies, the most suitable to this design is one that incorporates a small footprint with an integrated Complementary Metal Oxide Semiconductor (CMOS) backplane. The compact size along with the uniformity of drive offered by such a device will translate into better image quality than other LCD technologies. There has been progress in the projection display industry towards incorporating a single reflective LCD, primarily because of the lower cost and weight of single device systems. See U.S. Pat. No. 5,743,612. Of the LCD technologies, the reflective LCD with a silicon backplane can best achieve the high speeds required for color sequential operation. While this increased speed may not be as essential to printing as it is for projection display, the higher speeds can be utilized to incorporate additional gray scale and uniformity correction to printing systems.
While the reflective LCD modulator has enabled low cost digital printing on light sensitive media, the demands of high resolution printing have not been fully addressed. For many applications, such as imaging for medical applications, resolution is critical. Often, the resolution provided by a single reflection LCD modulator is insufficient. It then becomes necessary to create an image wherein multiple images are merged to create a single high-resolution image. Creating a merged image without artifacts along the borders, or in regions where image data may overlap, is desirable. While juxtaposing or spatially interweaving image data alone may have been attempted in previous applications, such a superposition of images with the use of reflective LCDs provides images of high quality without compromising the cost or productivity of the print engine. By utilizing polarization-based modulation, a print engine can utilize light already available in the optical system.
Juxtaposing or spatially interweaving image data has been attempted with some success in projection displays. Fergason, in U.S. Pat. No. 5,715,029, describes a method to improve resolution of a display by altering the beam path using a birefringent medium such as a calcite crystal or an electro-optic liquid crystal cell. For projection applications using a transmissive LCD, Philips Corporation deflects the beam path by using birefringent elements as shown in U.S. Pat. No. 5,727,860. Another method, using isotropic optical elements to juxtapose or spatially interweave images in a projection display using a transmissive LCD, is described in U.S. Pat. No. 5,626,411.
Modulator printing systems can incorporate a variety of methods to achieve gray scale. Texas Instruments employs a time delayed integration system that works well with line arrays as shown in U.S. Pat. Nos. 5,721,622, and 5,461,410. While this method can provide adequate gray levels at a reasonable speed, line printing Time Delayed Integration (TDI) methods can result in registration problems and soft images. Alternate methods have been proposed particularly around transmissive LCDs such as the design presented in U.S. Pat. No. 5,754,305.
It is desirable to increase the resolution of a photographic image, using available technology cost, and reduce reciprocity failure, while preserving adequate gray scale.
It is an object of the present invention to provide a method and apparatus for printing high resolution images using reflective LCD modulators. The present invention is directed at overcoming one or more of the problems set forth in the background of the invention.
Briefly summarized according to one aspect of the present invention, an apparatus for printing images comprising an illumination optics that provides uniform area light. A polarizing beamsplitter receives the uniform area light and redirects a first polarization state of the light to a first reflective LCD modulator which modulates the first polarized light on a site by site basis and reflects the first modulator light through the polarizing beamsplitter. A parallel plate tilted at a first angle spatially displaces the first modulated light along a first line normal to the polarizing beamsplitter. A print lens assembly images the first modulated light to a first position on a media plane to print a first image. The parallel plate is moved to a second position and a new image is printed
In one embodiment, the media is exposed in a color sequential manner with a two-dimensional color image. The LEDs are arranged in a two-dimensional pattern and are addressed in a series of pulses of varying amplitude and duration, in a color sequential manner to provide illumination of varying light levels to the reflective LCD modulator, thus extending the gray scale available through the reflective LCD modulator.
In another embodiment, an illumination optics lens is comprised of a collimating lens, a lenslet array, and a telecentric condensor lens, which provide uniform, telecentric illumination. Light of one polarization state incident on the polarizing beamsplitter is channeled in the direction of the reflective LCD modulator located at one facet of the polarizing beamsplitter.
Uniform light incident on the reflective LCD modulator is modulated on a site by site basis. Image data is displayed on the reflective LCD modulator as a series of frames corresponding to the illumination level and color. The voltage supplied to the reflective LCD modulator varies with the illumination wavelength. Light modulated by the reflective LCD modulator is passed through the polarizing beamsplitter and a print lens. Image data to the modulator contains independent data the superposition of which creates a composite image.
The print lens assembly is used to provide a high contrast magnified image at the image plane. At the image plane, multiple images from the reflective LCD modulator are generated in a color sequential manner and imaged on a two-dimensional area on the media. Upon completion of exposure of a given image, the media is advanced and the next image is exposed.
In a further embodiment, the print lens assembly is used to provide a high contrast demagnified image at the image plane.
In another embodiment, multiple reflective LCD modulators are imaged at more than one distinct location on the media by means of a prisms or mirrors.
In an additional embodiment, multiple reflective LCD modulators are imaged at more than one distinct location on the media by means of glass plates.
In a further embodiment, multiple reflective LCD modulators are imaged at more than one distinct location on the media by repositioning the print assembly.
A primary advantage of the present invention is the ability to produce high resolution images without reciprocity failures. Furthermore, a reflective LCD modulator is sufficiently fast so that a printer according to the present invention can create gray scale images without time delayed integration. For this reason, an apparatus according to the present invention can cover image artifacts due to image superposition without substantial mechanical or electrical complexity. The bulk of artifact reduction takes place in the software algorithms already designed for image correction.
The illumination system has been described with particular reference to a preferred embodiment utilizing LEDs as the light source. It is understood that alternative light sources and modifications thereof can be effected within the scope of the invention. The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below.